Inference and testDirection attribute instead of Inference(One|Two)Tailed [ following the Hackathon ]

As discussed at the hackathon/HBM 2014 with @chrisfilo, @nicholst, @nicholsn, @jbpoline, @gllmflndn and @tiborauer (see also #68). 

This is a proposal to replace `InferenceOneTailed`/`InferenceTwoTailed`  types by `Inference`with a `testDirection` attribute specifying if the test is one- or two-tailed. This will, in particular, ease querying across different inference activities.

(The same update is also implemented for conjunctions in this pull request).

This is similar to what was done for `StatisticMap` in Pull Request #80.

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Note: `nidm:OneTailedTest` (respectively `nidm:TwoTailedTest`) could re-use [one tailed test](http://bioportal.bioontology.org/ontologies/STATO/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FSTATO_0000286) (respectively [two tailed test](http://bioportal.bioontology.org/ontologies/STATO/?p=classes&conceptid=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FSTATO_0000287)) from STATO.